Polyamide compositions are used in a wide variety of applications because of their excellent physical properties, chemical resistance, and processability. Common applications include automotive parts and electrical and electronic parts. Though polyamides have good inherent toughness, low-elasticity rubber impact modifiers are often used to increase the toughness of polyamide compositions. However, the addition of these impact modifiers can reduce the stiffness of the resulting resin. Stiffness can be improved by the addition of reinforcing agents and fillers, particularly inorganic reinforcing agents (for example, glass fibers) and mineral fillers, but this measure can lead to further problems with wear on processing equipment, anisotropy, increased melt viscosities, and decreased hydrolysis resistance. Hence, a polyamide composition containing impact modifiers that has good stiffness without the need to add additional reinforcing agents and fillers would be desirable.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
It is known that impact strength can be markedly improved by adding an elastomeric material modified with reactive functional groups to polyamide resins. For example, a toughened polyamide blend is disclosed in U.S. Pat. No. 4,346,194, which contains a) 60 to 97 weight percent polyamide (a mixture of 66 nylon and 6 nylon) and b) 3 to 40 weight percent of a polymeric toughening agent selected from (i) an elastomeric olefin copolymer with carboxyl or carboxylate functionality or (ii) an ionic copolymer of at least one α-olefin and at least one α,β-unsaturated carboxylic acid, which can contain a ternary copolymerizable monomer, and which is at least partially ionized by neutralizing its acidic ingredients with a metallic basic salt.
Polyamide compositions have been disclosed in which melt viscosity and resistance to hydrolysis have been improved by the addition of polycarbodiimides. For example, a polycarbodiimide modified tractable polyamide product is disclosed in U.S. Pat. No. 4,128,599 with unique rheological properties and improved shear properties. It is disclosed that the polycarbodiimide functions as a bridging agent in which the carbodiimide group bridges the terminal COOH and the NH2 group in the polyamide.
U.S. Pat. No. 5,360,888 discloses a polyamide resin composition containing 0.1 to 5 weight aromatic polycarbodiimide that is stabilized to hydrolysis at high temperatures.
US patent application publication 2004/0010094 discloses a polyamide resin composition comprising aromatic or aliphatic polycarbodiimides in a ratio of 0.10 to 3.5 molar equivalents of carbodiimide groups to acid end groups in the polyamide.
The application of polyamide composition includes automotive parts as described above. In column-type electric power steering devices, for example, a small gear such as a worm and a large gear such as a worm wheel are used as a reduction gear mechanism and, after the rotation speed of an electric motor has been reduced and the power output has been augmented by transmission from the small gear to the large gear, the rotation is applied to a steering shaft, thereby torque assisting the steering operation. In electric power steering devices used in lightweight four-wheel automobiles or in the usual passenger vehicles, in particular comparatively small-size passenger vehicles, the so-called resin gears have been generally used for at least one gear of the small gear and large gear, in particular for the large gear, in which at least the teeth thereof are made from a resin instead of the conventional metal, with the object of reducing the noise level by decreasing the tooth impact noise in the reduction gear mechanism, decreasing the weight, and reducing the sliding resistance.
More specifically, resin gears having a composite structure of an annular metal core that is coupled to a steering shaft so as to enable the integral rotation therewith and an annular gear body made from a resin, having teeth on the outer periphery thereof, and formed so as to surround the outer periphery of the core have been widely used as large gears for gear reduction mechanisms of electric power steering devices. Furthermore, polyamide resins such as, for example, MC (monomer casting) Nylon, Polyamide 6, Polyamide 66, and Polyamide 46 have been generally used as the base resins for forming the gear body. In recent years, the possibility of using resin gears as large gears for reduction gear mechanisms also in electric power steering devices of larger automobiles has been more studied compared to before. Furthermore, a demand has been recently created for electric power steering devices that are smaller in size and weight than the conventional devices, regardless of the automobile size, with the object of producing automobiles with better fuel efficiency that cause less environmental pollution.
However, the problem associated with such conventional resin gears is that when they are incorporated in electric power steering devices of large automobiles or electric power steering devices reduced in size to decrease fuel consumption, the required endurance performance cannot be sufficiently ensured and the gears are fractured within a comparatively short interval. This is because the power output of the electric motor has to be increased and the torque transmitted to the reduction gear mechanism increases as the automobile becomes larger in size. Furthermore, as the size of the electric power steering device is reduced, it becomes difficult to take such measures as increasing the module and decreasing the surface pressure in large gears, and the surface pressure transmitted from small gears to large gears tends to increase. Presently, the conventional resin gears are unable to meet fully the requirement to increase the power output of reduction gear mechanisms.
Accordingly, it is desirable to provide resin gears with both the high rigidity corresponding to a transition to a higher power output of reduction gear mechanisms, in particular, the rigidity necessary to maintain a high strength in a high-temperature environment in which the electric power steering devices are used, and the high toughness preventing the resin gear from fracturing easily under applied stresses. It is well known that the rigidity of resin compositions can be increased by blending reinforcing fibers such as glass fibers with a polyamide resin serving as a base resin. However, the resultant problem is that the toughness of the resin composition decreases significantly. Yet another problem is that the resin gears formed from such resin compositions have the so-called attack ability and the teeth of the metal gears assembled therewith are easily damaged or worn. Moreover, the fine metal powder (wear powder) produced from the damaged and worn-out metal gears easily cut or damage the teeth themselves. For this reason, resin compositions of a non-reinforced type that contain no reinforcing fibers are preferred for forming resin gears. Various research have been conducted to improve properties of non-reinforced resin compositions.
For example, it is known that the toughness of a resin composition can be increased by blending an elastic material with a polyamide resin serving as a base resin. However, because the rigidity is decreased, the resin gear formed by using such resin composition can be easily deformed. The resultant problem is that, for example, when the resin gear is used as a large gear, and a torque is applied by the rotation of an electric motor via a small gear, the large gear deforms, the transmission ratio of the torque from the small gear is reduced or the rotation is delayed and the engagement with the small gear is easily degraded. Yet another problem is that, in addition to susceptibility of such resin gears to instantaneous deformation, a creep deformation also can easily occur therein, and the creep deformation easily decreases dimensional stability and causes fluctuations of backlash in engagement with the small gears.
It is also known that increasing the molecular weight of polyamide resins can increase the toughness, while preventing the rigidity from decreasing. However, for example, when various additives are blended with a polyamide resin and a molding material (pellets, etc.) for injection molding is manufactured after kneading in a state where the resin is heated to a temperature equal to or higher than the melting point of the polyamide resin and melted, or when a resin gear is produced by heating again the manufactured molding material to a temperature equal to or higher than the melting point thereof and melting in a cylinder of an injection molding apparatus and then injecting the resin into a metal mold linked to the injection molding apparatus, for example, into a shaped cavity corresponding to the shape of the gear body in the case of resin gears of a complex structure, the decomposition of the polyamide resin is enhanced and the molecular weight thereof decreases under the effect of moisture adsorbed by the polyamide resin and heat during melting. The resultant problem is that even if a polyamide resin with an increased molecular weight is used, the toughness of the resin gear manufactured by the above-described process cannot be increased.
It is known that various properties of resin compositions can be improved by blending a polycarbodiimide compound with a polyamide resin. For example, Japanese Patent Application Laid-open No. H6-16933 (claim 1, paragraph No. 0001, 0004-0007) describes that if a polymer aromatic polycarbodiimide is blended at a ratio of 0.1 to 5 wt. % with a polyamide resin, then stability of the resin composition against hydrolysis, in particular, in acidic media can be improved. Furthermore, Japanese Patent Application Laid-open No. H9-194719 (claim 1, paragraph No. 0005-0008, 0031, 0062) describes that if 1-300 parts by weight of at least one elastic material selected from the group including a polyamide elastomer, a polyester elastomer, an acrylic rubber, a silicone rubber, and a fluorine-containing rubber, a modified polyolefin rubber, and a modified rubber is blended with a total of 100 parts by weight of 30 to 99 wt. % a thermoplastic resin such as a polyamide resin and 70 to 1 wt. % polycarbodiimide, then heat resistance and impact resistance of the resin composition can be improved. Furthermore, Japanese Patent Application Laid-open No. H11-343408 (claim 1, paragraph No. 0002-0003, 0006-0007, 0012-0014) describes that if an aliphatic carbodiimide compound is blended with a polyamide resin, then stability of the resin composition against hydrolysis, oil resistance, and resistance to metal halides can be increased.